do we need intumescent seals?

[daisy12345]

Hi all, we live in a three-storey house (with loft conversion) but the loft was done ages ago so doesn't meet all the latest building regs. We don't need to retrofit to meet the building regs as I understand, but do want to change the old doors to fire doors to improve fire safety. We got some free old fire doors that are the right size on freecycle, and plan to replace the old doors with those. Now the question is: What are the things that we can do to try to make it as fire resistant as possible please? Using fire-rated hinges is one. Installing intumescent strips? I read on screwfix that "As a guide, only use 15mm seals on new door application and 10mm seals for replacement / retrofitting." so we should get the 10mm seals? And anything else please?

Any comments are much appreciated!